Janette Duncan, estimable Scottish Fiddler and member of Alasdair Frasier's San Francisco-based ensemble, as well as former member of my band Greenhouse, has been using octave strings on her second fiddle for several years now, with no adverse effect on the instrument.
The sound - especially when amplified - is awesome! Try it, you'll like it. There is a source for the strings here in Santa Rosa, CA. PM me if you'd like me to find out the info.